Mostly incoporating feedback from @gregorycollins on my original pull request. I
have:
* Simplified the documentation for `wrapSite` to be more direct, and removed a
very contrived example.
* Slightly amended the documentation for runSnaplet as it doesn't return a set
of messages at all, but rather a single Text value.
* Rewrote the documentation for serveSnaplet as requested.

* wrapSite now clarifies that it wraps the base snaplet, not just the current
snaplet.
* runSnaplet now correctly shows the string "devel", instead of trying to link
to the `devel` identifier.
* serveSnaplet clearly documents both parameters, providing a hint that
defaultConfig can be used. Also removed the FIXME comment.

ISessionManager typeclass functions are low level are not intended for
use by end users of Session or any of its backends. Since some
functions have names that clash with Prelude exports, it is better to
expose them from a separate module that's meant to be used by backend devs.